The present invention relates to an aluminum sheet material for lithographic printing plates that prevents swelling of a protective layer, particularly due to aluminum carbide, when used to produce a lithographic printing original plate.
A lithographic printing original plate produced by forming a lipophilic photosensitive resin layer on a hydrophilic support formed of an aluminum sheet material has been widely used. A desired printing plate has been normally obtained by performing mask exposure via a lith film, and dissolving and removing the non-image area.
In recent years, digitization technology that electronically processes, stores, and outputs image information using a computer has been widely used. A variety of new image output methods that deal with such digitization technology have been put to practical use. As a result, computer-to-plate (CTP) technology that directly produces a printing plate by scanning light with high directivity (e.g., laser light) based on digitized image information without using a lith film, has been used.
When producing a lithographic printing original plate using the CTP technology, a printing plate aluminum sheet material is subjected to a surface-roughening process (e.g., graining) and a sulfuric acid anodizing process, and a photosensitive resin layer and a protective layer are formed on the surface of the aluminum sheet material, followed by heating and exposure. However, the protective layer of the lithographic printing original plate may swell and cause an image defect (e.g., image loss) due to exposure failure. It was confirmed that an image defect (e.g., image loss) due to exposure failure occurs when swelling with a diameter of 50 μm or more occurs.
WO2007/093605 A1 discloses an aluminum sheet material for lithographic printing plates in which the aluminum carbide content is limited to less than 10 ppm on the assumption that the protective layer swells when aluminum carbide contained in the aluminum sheet material reacts with steam inside the protective layer to produce methane gas.